Water tends to convert metals (such as mild steel) to their oxide
states. The corrosion is a result of dissolved gases, improper pH control or
formation of differential aeration cells under deposits. A localized effect of
corrosion results in built up of holes; the phenomenon known as pitting.
Failures of this type can be catastrophic, leading to costly downtime for
repairs and equipment replacement and even total plant shutdown.
1. What is corrosion?
Corrosion is an electrochemical process by which a
metal returns to its natural state i.e. forms oxide in contact with oxygen.
2. How does corrosion take place?
For corrosion to occur, a corrosion cell, consisting
of an anode, a cathode and an electrolyte must exist. Metal ions dissolve into
the electrolyte (water) at the anode. Electrically charged particles are left
behind. These electrons flow through the
metal to other points (cathodes) where electron-consuming reactions occur. The
result of this activity is the loss of metal and often the formation of a deposit.
3. Which materials are susceptible to corrosion?
Mild steel is a commonly used metal in the cooling
water system that is most susceptible to corrosion. Other metals in general,
such as copper, stainless steel, aluminum alloys also do corrode but the
process is slow. However in some waters and in presence of dissolved gases,
such as H2S or NH3, the corrosion to these metals is more
severe & destructive than to mild steel.
4. What types of corrosion exists in cooling water systems?
Many different type of corrosion exist, but the most
common is often characterized as general, localized or pitting and galvanic.
ü General attack: exists when the corrosion is
uniformly distributed over the metal surface. The considerable amount of iron
oxide produced contributes to fouling problems.
ü Pitting attack: exists when only small area of
the metal corrodes. Pitting may perforate the metal in short time. The main
source for pitting attack is dissolved oxygen.
ü Galvanic attack: can occur when two different
metals are in contact. The more active metal corrodes rapidly. Common examples
in water systems are steel & brass, aluminum & steel, Zinc & steel
and zinc & brass. If galvanic attack occurs, the metal named first will
corrode.
5. What water characteristics affect corrosion?
§ Oxygen
and other dissolved gasses
§ Dissolved
or suspended solids
§ Alkalinity
or acidity (pH)
§ Velocity
§ Temperature
§ Microbial
activity
6. How does oxygen affect corrosion?
Oxygen dissolved in water is essential for the
cathodic reaction to take place.
7. How do dissolved or suspended solids affect corrosion?
Dissolved solids can affect the corrosion reaction by
increasing the electrical conductivity of the water. The higher is the
dissolved solids concentration, the greater shall be the conductivity and more
is the likelihood of corrosion. Dissolved chlorides and sulphates are
particularly corrosive.
8. How does alkalinity or acidity affect corrosion?
Acidic and slightly alkaline water can dissolve metal
and the protective oxide film on metal surfaces. More alkaline water favors the
formation of the protective oxide layer.
9. How does the water velocity affect corrosion?
High velocity water increases corrosion by
transporting oxygen to the metal and carrying away the products of corrosion at
a faster rate. When water velocity is low, deposition of suspended solids can establish
localized corrosion cells, thereby increasing corrosion rates.
10. How does temperature affect corrosion?
Every 25-30°F
increase in temperature causes corrosion rates to double. Above 160°F,
additional temperature increases have relatively little effect on corrosion
rates in cooling water system.
11. How does microbial growth affect corrosion?
Microbial growths promote the formation of corrosion
cells in addition; the byproducts of some organisms, such as hydrogen sulphide
from anaerobic corrosive bacteria are corrosive.
12. What methods are used to prevent corrosion?
Corrosion can be prevented or minimized by one or
more of the following methods:
§ When
designing a new system choose corrosion resistant materials to minimize the
effect of the aggressive environment.
§ Adjust
pH.
§ Apply
protective coatings such as paints, metal plating, tar or plastics
§ Protect
cathodically, using sacrificial metals.
§ Add
protective film- forming chemical inhibitors that the water can distribute to
all wetted parts of the system.
13. How do chemical corrosion inhibitors work?
Chemical inhibitors reduce or stop corrosion by
interfering with corrosion mechanism. Inhibiting usually affect either the
anode or the cathode.
ü Anodic corrosion inhibitors establish a
protective film on the anode. Though these inhibitors can be effective, they
can be dangerous, if sufficient anodic inhibitor is present, the entire
corrosion potential occurs at the unprotected anode sites. This causes severe
localized (or pitting) attack.
ü Cathodic corrosion inhibitors form a
protective film on the cathode. These inhibitors reduce the corrosion rate in
direct proportion to the reduction of cathodic area.
ü General corrosion inhibitors protect by
filming all metal surfaces whether anodic or cathodic.
14. What inhibitors are commonly used for cooling water
systems?
ü Mainly anodic: Chromates, Nitrites,
Orthophosphates, and Silicates
ü Mainly cathodic: Bicarbonates, Metal cations,
Polyphosphates
ü General: Soluble oils, other organics
Yes. The choice of treatment is basically a mater of
economics. In a once-through system, a very large volume of water passes
through the system only once. Protection can be obtained with relatively few
parts per million (ppm) of treatment because the water does not change in
composition significantly while passing through the equipment.
In an open re-circulation system, more chemical may
be present because the water composition changes significantly through the
evaporation process. Corrosive and scaling constituents are concentrated.
However, treatment chemicals also concentrate by evaporation, therefore, after
the initial dosages only moderate dosages will maintain the higher level of
treatment needed for these systems.
In a closed re-circulation system, water composition
remains fairly constant. There is very little loss of either water or treatment
chemical. The best form of treatment recommendation for closed water system
includes the dosage of film forming inhibitors such as nitrites and molybdate.
16. What are the effects of corrosion on the re-circulation
system?
§ Damage
to pump seals
§ Plugged
lines
§ Loss
if heat transfer efficiency
§ High
maintenance & replacement costs
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